The present invention relates to fission reactors and, more particularly, to a refueling mast for a fission reactor. A major objective of the present invention is to provide a refueling mast which is suitable for a boiling water reactor and which is both economical and durable.
Nuclear fission reactors promise to provide abundant energy with far less strain on the environment than fossil fuels. However, since fissionable fuels are hazardous materials, great care must be taken in their transit. This care is especially important within a reactor complex, where fuel elements are transferred between storage area and reactor core. The transfer must be mechanized so that operators are not exposed to radiation.
In some boiling water reactor complexes, fuel elements are in the form of rods which are inserted into a core where the heat-generating fission reaction takes place. Spent fuel elements are transferred to a storage area and fresh fuel elements are transferred from the storage area to the core. The reactor core is situated in a reactor vessel and submersed in water which is circulated to provide for heat transfer. The storage area is also submersed in water in a separate tank, in part because water shields radiation emitted from a fuel element. Typically, the storage tank and the reactor vessel are separated by a barrier, e.g., of concrete.
Transfer of fuel elements can be effected using a trolley, a movable bridge which is moved along tracks and spans the region including the reactor vessel and the storage area. A fuel grapple is used to engage fuel assemblies so that they can be laterally transferred from one area and to release them once they are securely positioned at their destination.
Fuel element transfer requires vertical as well as lateral transfer. Typically, the fuel elements in storage are at a different depth than the fuel elements in the core. Furthermore, fuel elements must be lifted over the barrier between the storage and reactor vessel areas. A gate is typically placed in this barrier, but it provides an opening which is sufficient only to avoid a need to lift a fuel element in transit out of the water.
Vertical movement can be effected using a "refueling mast", typically a vertically telescoping assembly including nested tubes. The outer tube is fixed, nested inner tubes extend downward from and retract into the outer tube. To transfer fuel elements in and out of the core, the refueling mast must permit precise positioning and orienting of the grapple even when the mast is fully extended. In other words, the mast should be sufficiently rigid so that the drag induced by relative movement through water does not bend or twist the mast significantly. This rigidity is desirable to reduce settling time after lateral moves and to improve the operator's "feel" and control over fuel assembly position. In addition, the refueling mast should be sufficiently strong to resist impact damage during possible accidental collisions with vessel components. Such damage can require repair or replacement of the refueling mast, in either case, causing expense and down-time for the reactor complex.
Some refueling masts incorporate telescoping steel tubes having square cross-sections. The walls of the tubes can be 1/2" to 1/3" thick, providing strength and rigidity. The square cross sections contribute to torsional rigidity. Because precision, corrosion-resistant, square tubing is not widely available, these refueling masts are very expensive. In addition, the several thick square tubes constituted very massive refueling mast.
A more lightweight and economical refueling mast has been fabricated by welding small cylindrical trusses together, defining a skeletal "tube" of triangular cross-section. While formed of readily available cylindrical tubing, these refueling masts do incur the added cost of hand welding the trusses together. More seriously, these trussed refueling masts are readily damaged by accidental impacts and require repair and/or replacement more frequently than is desirable.
What is needed is a more economical refueling mast which provides the strength required to resist impact damage, and the translational and torsional rigidity to limit bending and twisting of the mast due to hydrodynamic drag. A method is desired for forming such a mast using readily available components and relatively inexpensive assembly procedures.